It is known that an electric power system is planned and designed for normal, as well as disturbed, operation with respect to fixed dimensioning criteria relating to system quantities and power transmissions. The angular difference between voltage vectors in various parts of the electric power system is one such quantity. This angular difference is a measure of the transmitted active power and a measure of the margin of stability of the system.
When a disturbance arises in an electric power system, for example a short circuit or a ground fault, which leads to disconnection of one or more branch elements, for example a line, for the arc at the site of the fault in case of a transient fault to expire and the fault disappear, automatic reclosing is often used, with amplitude, frequency and angular position control of the voltage across the circuit breaker in question [Kundur, Power System Stability and Control, ISBN 0-07-035958-X, p. 922] In order for reclosing to be allowed, the differences between the values for these respective quantities, on both sides of the circuit breaker, must be smaller than certain predetermined values/criteria. The method is called controlled reclosing with synchronism control. The purpose of rapidly reclosing the line, after the arc has been extinguished and the site of the fault deionized, is to promote stability and reduce the risk of an out-of-phase situation; alternatively, a higher stationary-operation transfer of power is allowed. If the difference in amplitude, frequency or phase position between the voltages on both sides of the circuit breaker should be too great and connection of the circuit breaker is performed, there is a risk of such high transient stresses that equipment may become damaged and/or that the operation of the electric power system is further disturbed. Synchronism control is used to block the reclosing in those cases where the voltages on both sides of the circuit breaker differ by more than a certain value with respect to amplitude, frequency or phase position.
The angular difference between the voltage vectors on both sides of the circuit breaker is compared, in the present equipment, with a fixed set value only. In the case of large power transmissions prior to a fault and disconnection of the line under consideration, the angular difference between the ends of the lines is relatively great also during stationary operation. After the circuit breakers, initiated by the relay protection devices, have disconnected the line at its end points, the power flow in a masked system is forced to take other directions, which further increases the transfer angle. The automatic reclosing system is started at the two ends of the line by the released line protection devices. Thereafter, after checking that the line is dead, the faulty line is first reclosed at the opposite end. The angular difference across the breaker poles of the circuit breaker under consideration will then be relatively great since the angle is not changed along the energized, but unloaded, line. Consequently, there is a considerable probability that too great an angular difference exists/is measured across the poles of the circuit breaker under consideration, and the reclosing operation will then be blocked.
A recent example of the sequence of events described above is the major blackout in Italy on 28 Sep. 2003 [Interim Report of the Investigation Committee on the 28 Sep. 2003 Blackout in Italy, UTCE Report—27 Oct. 2003], where a failing reclosing contributed to the uncontrolled sequence and propagation of the disturbance. The limit to the angular difference across the breaker poles, to allow reclosing, was at that time set at 30°, which blocked the reclosing, resulting in the most extensive electricity blackout in the world so far.